The present invention relates to a hermetically sealed flywheel for use in a canned motor pump or a like application. The present invention also relates to a method of manufacturing such a flywheel, which insures the integrity of the flywheel assembly notwithstanding the adverse conditions of its use.
Centrifugal pumps having flywheels are well known, the flywheel being incorporated to mechanically store potential energy during operation of the pump in order to maintain rotation of the pump in the event of loss of electric power. The flywheel is generally a metal disk having a relatively high mass and being precisely attached to or mounted on the motor shaft for rotation therewith, the inertia of which keeps the shaft rotating for some period of time after de-energization of the motor. In nuclear reactors, such flywheels are used to maintain coolant circulation through the reactor core after the coolant pumps are shut off pursuant to a reactor "trip", since the nuclear fuel continues to give off substantial amounts of heat after such a shutoff and since the resulting protraction of pump shaft rotation prolongs the circulation of coolant through the core.
Pressurized water reactor (PWR) coolant pumps generally include a pump and motor which are separated by a complicated shaft seal system, the seal being used as a part of the reactor coolant system pressure boundary. The seals are subject to about a 2,500 psi pressure differential between the reactor coolant system and the containment atmosphere. There is a possibility that these seals may fail, and cause a non-isolable leak of primary coolant ranging in size from very small to fairly large. Such a leak can result in damage to the pump and other equipment, and can necessitate a costly clean-up operation.
Canned pumps have been used in nuclear reactor plants for some time to obviate the need for such a shaft seal arrangement since the entire pump, including bearings and rotor, are submerged in the pump fluid. Therefore, the use of a canned pump reduces the potential for a loss of coolant accident (referred to in the art as a "LOCA"). Exemplary canned motor pumps are described in U.S. Pat. Nos. 3,450,056 and 3,475,631. In pressurized water reactors, continued rotation of these pumps upon loss of electric power is provided by electro-mechanical means, generally in the form of motor-generator sets having flywheels incorporated therein. The motor-generator set is generally located outside of the reactor containment for accessibility purposes, the electricity being transmitted from the generator to the pump motor through containment wall penetrations. In the event of a loss of electric power to the motor-generator set, the flywheel maintains rotation of the generator for some period of time, which continues to provide power to the motor pump. However, due to the lack of mechanical inertia in the pump itself, any localized failure of the pump or its controls may prevent the pump from extended coast-down. In addition, due to the necessity for extra equipment, this option is fairly expensive both in capital cost and in operation and maintenance costs.
A flywheel within a canned or wet winding pump has been utilized. However, the fluid friction losses resulting from spinning a large, high mass flywheel through the fluid contained in the pump casing are substantial. The outer surfaces of the flywheel attempt to frictionally pump the surrounding fluid, while the casing surrounding the flywheel inhibits fluid flow. Therefore, turbulent vortices form that cause highly distorted fluid velocities which in turn applies substantial drag on the flywheel. This drag is a function of the speed and area of the surface of the flywheel, both of which increase with the radius of the flywheel, such drag being commonly understood to increase with about the fifth power of the diameter and about the cube of the angular velocity.
One arrangement to reduce flywheel drag is disclosed in U.S. Pat. No. 4,084,924 to Ivanoff et al. This patent describes a canned pump having a flywheel and a free-wheeling shroud rotatable relative to the shaft and the flywheel. The shroud encompasses the flywheel but is spaced apart therefrom and includes passages for ingress and egress of liquid into and out of the space between the flywheel and the shroud. The disclosure envisions that the shroud will rotate at some angular velocity between zero and the velocity of the flywheel, thereby creating two pumped fluid layers, one (between the flywheel and the shroud) being pumped by the flywheel, and the other (the layer outside the shroud) being pumped by the shroud. The lower relative angular velocity between the rotating surfaces therefore results in lower total drag.
U.S. Pat. No. 4,886,430, issued Dec. 12, 1989 to the present inventor (as a co-inventor) describes a canned pump having a high inertia flywheel immersed along with the rotor in the pumped fluid (e.g., reactor coolant water), wherein the drag losses associated with the flywheel spinning in the pumped fluid are minimized by surrounding the flywheel with bearing means, such that the flywheel serves also as a high inertia journal bearing. The entire contents of U.S. Pat. No. 4,886,430 is hereby incorporated by reference. The flywheels used in such applications should have as high an inertia as possible while having dimensions as small as possible to reduce drag. In order to accomplish these conflicting objectives, it is desirable to manufacture the flywheel from a metal of high specific gravity. Expense limits the use of gold, palladium and tungsten. Additionally, tungsten is brittle. And, while spent (depleted) uranium is a highly dense material that is relatively inexpensive, it will corrode readily when exposed to moisture. While it is proposed in the just mentioned patent to hermetically seal such a uranium ring in a "can" of non-corrosive metal, thus far no such sealing structure has been developed which is 100% reliable in maintaining the integrity of the seal during periods of prolonged rigorous use of the flywheel, e.g., as a journal bearing in a canned motor pump. The stresses associated with such rigorous use may cause weld seams to break, as may the thermal differential expansion between the uranium ring and the "can" of non-corrosive metal. Thus, there is a need for a flywheel that is as dense as spent uranium but which is not subject to corrosion.